Layer class abstract interface

Middleware that can be written as a const expression.

middleware: on @Controller and on a verb only accepts const values, so there has to be a way to get from one of those to a shelf Middleware. Dust emits const RateLimit(perMinute: 30).toMiddleware(). The name follows axum's Layer, and Router.layer takes either this or a bare shelf Middleware.

final class RateLimit implements Layer {
  const RateLimit({required this.perMinute});

  final int perMinute;

  @override
  Middleware toMiddleware() => ...;
}